1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for producing the same. More particularly, it relates to a reflection-type liquid crystal light valve for a liquid crystal projection display device.
2. Description of the Related Art
Recently, a super high definition active matrix projection LCD (liquid crystal display) has attracted widespread attention for its ability to meet high definition and large-screen display requirements and its potential to replace CRTs (cathode ray tubes). Generally, an active matrix LCD comprises: a semiconductor substrate on which switching elements and display electrodes connected to the switching elements are formed; and a counter substrate on which counter electrodes are formed above the semiconductor substrate, where a liquid crystal fills the region between the semiconductor substrate and the counter substrate (the cell gap).
A projection LCD device comprises a light source, a light valve, a screen, an optical filter, and a projection lens. There are two types of light valves which are used for the LCD device. One is a transmission-type liquid crystal light valve for projecting an image on a screen by letting in the light from the light source, and the other is a reflection-type liquid crystal light valve for projecting an image on the screen by reflecting the light from the light source.
FIG. 11 is a schematic sectional view of one example of a reflection-type liquid crystal light valve. As shown in FIG. 11, the reflection-type liquid crystal light valve 200 has: a transistor 204 formed in a semiconductor substrate 202; a counter substrate 206 provided on a light-incident side; a plurality of light-reflecting films 210 connected electrically to the semiconductor substrate with a space 208 therebetween; and liquid crystal 212 filled in a cell gap between the light-reflecting films and the counter substrate. Over the transistor 204, a light-reflecting film 210 which acts as a reflecting electrode is formed on an interlayer insulating layer 214 and the like. The liquid crystal 212 is filled in a cell gap between these light-reflecting films 210 (which films act as reflecting electrodes), and the counter substrate 206 consisting of a counter electrode 216 and a glass cover plate 218. The light-reflecting films 210 which act as reflecting electrodes are formed for each pixel region including each transistor 204. Between mutually adjacent light-reflecting films 210, a space 208 is formed. In such a liquid crystal light valve, a voltage between the light-reflecting films 210 and the counter electrode 216 is controlled in accordance with a display signal, and thereby an array of the liquid crystal 212 placed therebetween is properly controlled.
In the case of a reflection-type liquid crystal light valve, incident light passes through the space between the reflecting electrodes and is transmitted into the interlayer insulating layer, and then propagates through the interlayer insulating layer by multipath reflections. When this light comes near the transistor, it causes a leakage current as a result of the photoconductive effect, and thus degrades the contrast of the LCD. The photoconductive effect becomes more pronounced as the patterning of pixels becomes finer. Therefore, like the LCD light valve 200 shown in FIG. 11, a liquid crystal light valve is generally provided with a light-blocking layer 220 and an insulating layer 222 formed on the light-blocking layer 220 in order to avoid the above-mentioned photoconductive effect and to block the incident light.
In various steps of forming the above-mentioned semiconductor substrate, residual reactive products generated in the deposition process used, such as a CVD (Chemical Vapor Deposition) process, and foreign particles such as metal powders and organic pieces generated in the course of the operation of an apparatus, may all be left and deposited in the apparatus, adhering to the surface of the substrate. For example, foreign particles may be generated during a photolithographic process and adhere to a surface of a resist layer or a metal layer. FIG. 12(a) shows a case where a foreign particle 230 adheres to an insulating layer 214 in the liquid crystal light valve. If the light-blocking layer 220 is formed on the insulating layer 214 with a foreign particle thereon, a protrusion is formed on its surface as shown in FIG. 12(b), and thus the smoothness of the surface is spoiled. As a result, the insulating layer 222 formed on the light-blocking layer 220 is substantially thinned near the foreign particle, and therefore insulation between the light-blocking layer 220 and the reflecting electrode 210 cannot be assured.
However, there is a method to keep the insulation. In that method, after the light-blocking layer 220 is formed, the protrusion including a foreign particle is removed by a physical method such as CMP (chemical mechanical polish) to flatten the surface of the light-blocking layer 220, and then the insulating layer 222 is formed on the light-blocking layer 220, as shown in FIG. 12(c).
However, since the light-blocking layer itself is also removed by this method, an underlying transistor fails to work properly due to incident light, and thus pixel failure occurs. More specifically, since the light shield and light-reflecting electrode are formed almost all over the semiconductor substrate, the foreign particle causes a short circuit between the light-blocking layer and a reflecting electrode or light leakage causes the malfunction of a transistor, which leads to the pixel failure.
Published Japanese Patent Application No. 8-328034 discloses a liquid crystal light valve comprising a plurality of light-blocking layers to inhibit a leakage current due to the photoconductive effect. For example, as shown in FIG. 13, two metal layers 304 and 306 are formed by sandwiching the insulating layer 222 therebetween, so that they can shield the surface of the semiconductor substrate from the light incident from a space 208 between the reflecting electrodes 210. These metal layers 304 and 306 are each provided with a slit 308. Since these slits 308 are staggered, the incident light is reflected by either of metal layers 304 or 306, so that it cannot reach the semiconductor substrate.
As the number of light-blocking layers such as metal layers 304 and 306 increases, more shielding effects are produced. However, since a step of forming the metal layer 304 or 306 has to be repeated, the process becomes more complicated and increases in number, which leads to higher manufacturing costs.
Published Japanese Patent Application No. 9-33952 discloses a liquid crystal light valve 400 shown in FIG. 14, which can solve the above problems. In this liquid crystal light valve 400, the space 402 between the light-reflecting films 210 (reflecting electrodes) are filled with a light-blocking insulating layer 404 consisting of two color resists selected from red, blue, and green. The light of wavelengths in a specific region may pass though the space 402 between the light-reflecting films 210, however, the light is completely blocked by the light-blocking insulation layer 404 before it reaches an MOS (Metal Oxide Semiconductor) transistor and thus the leakage current can be prevented. The light-reflecting films 210 of the liquid crystal light valve described in the above patent application should be thick enough to form two colored resist layers of uniform thickness, however, it may often cause a problem in terms of the formation of thinner layers.
Moreover, as the demand for higher definition and higher production efficiency increases, the simplification of processes, the reduction of power consumption of pixels, and high-speed operation are always required in the process of forming a liquid crystal light valve.
Accordingly, it is an object of the present invention to provide a light-reflection type liquid crystal light valve for a projection type liquid crystal display device, which can prevent an underlying transistor from malfunctioning due to a leakage of light incident from a light source.
Another object of the present invention is to provide a liquid crystal light valve capable of preventing shorts between a light-blocking layer and a reflecting electrode, which is caused by a foreign particle.
Still another object of the present invention is to achieve a simple process for manufacturing a semiconductor substrate of a liquid crystal light valve, low power consumption, and higher-speed operation by utilizing an electric capacitance obtained by the formation of a light-blocking layer.
The above and other objects of the present invention are achieved by a liquid crystal light valve according to the present invention having:
a plurality of light-reflecting films with a space therebetween;
a semiconductor substrate connected electrically to the light-reflecting films;
a counter substrate provided on the incident-light side;
liquid crystal filled in a cell gap between the light-reflecting films and the counter substrate;
an electric circuit formed in the semiconductor substrate for applying voltage to the light-reflecting films and counter electrodes formed on the counter substrate;
a light-blocking layer formed below the light-reflecting films;
a first insulating layer formed between the light-blocking layer and the electric circuit;
a second insulating layer formed between the light-reflecting films and the light-blocking layer;
a stud for electrically connecting the electric circuit and the light-reflecting films, insulated from the light-blocking layer;
light shields provided on the light-blocking layer formed below the light-reflecting films for blocking the incident light from the electric circuit; and
a third insulating layer formed between the light shields and the light-reflecting films.
Since the light shields block the light incident from a space between the light-reflecting films and the light-blocking layer further blocks the light, the above-described liquid crystal light valve according to the present invention is capable of blocking the light effectively.
Further, the third insulating layer is capable of ensuring insulation between the light shields and the light-reflecting films and also functioning as a capacitor for storing electric charges. For this reason, the third insulating layers can hold a part or all of the capacitance necessary to drive liquid crystal in each pixel.
In the liquid crystal light valve of the present invention, especially in the reflection-type liquid crystal light valve used in the projection-type liquid crystal display device, the structure of the light shield for shielding the underlying transistor from the incident light from the space between the light-reflecting films is different from that of the conventional art. In the present invention, stud-like light shields are each formed between the light-reflecting layers and the light-blocking films and along the inside peripheral portion of the light-reflecting layers. Since the incident light from the space between the light-reflecting films can be effectively blocked, malfunction of an underlying transistor and pixel failure never occur, but yield and production efficiency can be increased.
Since the insulating layer between the light shield and the light-reflecting film also acts as a capacitor, stable voltage can be applied to the liquid crystal by storing a part of electric charges in this insulating layer.
The present invention also discloses a method for producing the liquid crystal light valve described above In this method for producing the liquid crystal light valve, light shields can be formed in the same step in which the stud is formed, so that remarkable light-blocking effects can be obtained without adding any complicated steps to the conventional steps of producing a liquid crystal display device.
The method for producing the liquid crystal light valve according to the present invention comprises no additional complicated processes, but the liquid crystal light valve of the above configuration can be produced through almost the same steps as the conventional ones.
Alternatively, the capacitor between the light shield and the light-reflecting film can hold all the storage capacitance for a pixel. In this case, there is no need of additional storage capacitor, so that the manufacturing processes can be substantially simplified.
In addition, the present invention also discloses a projection type liquid crystal display device using the above liquid crystal light valve.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.